The present invention relates to novel high melting point crystalline polyamides, and to compositions containing them, as well as to objects, for example molded or extruded objects, obtained from these polyamides; it also concerns the process for producing these polyamides.
Polyamides are currently employed in various types of industry. Such polyamides needs to possess various properties enabling them to be used under increasingly severe conditions. Thus, there is a demand for polyamides having excellent thermo-mechanical properties. Such polyamides must thus exhibit these properties at high temperatures, typically above or equal to 250.degree. C., in other words their melting temperature T.sub.M must be higher than or equal to 250.degree. C. Mechanical properties such as flexural modulus, hot deformation temperature (HDT), impact resistance, and, in particular, a high modulus of rigidity, among others, are the mechanical properties that the copolyamide should possess.
The mechanical properties of the polyamides are typically linked to their crystallinity, and this hence should be high. Moreover, these polyamides should also have a high glass transition temperature, conventionally above or equal to 120.degree. C. Moreover, as the polyamides are made up by several monomers, it is desirable for the thermo-mechanical properties of the polyamides to remain high over the whole range of composition of the polyamide.
Those skilled in the art will hence look for copolyamides having:
a) a melting temperature T.sub.M higher than or equal to 250.degree. C.; PA0 b) high crystallinity; PA0 c) good mechanical properties such as a high modulus of rigidity, PA0 d) a glass transition temperature T.sub.g higher than or equal to 120.degree. C.; PA0 e) maintenance of these properties throughout the composition range.
It is known that copolyamides contain a significant amount of the 6,T unit (a unit derived from condensation of hexamethylene diamine and terephthalic acid) leading to copolyamides able to withstand more stringent thermomechanical conditions, meaning that their melting temperature is in the range of 240.degree. to 360.degree. C. Usually, for reasons particular to the process and taking account of the temperatures at which the products deteriorate, specialists in the field look for copolyamides that include an appreciable amount of 6,T, and have melting temperatures in the range between 290.degree. and 320.degree. C. Higher melting temperatures make both the synthesis and transformation of these copolyamides difficult, for the reasons discussed above.
For example, introducing the 6,I unit, even if it does allow 6,I/6,T copolyamides to be obtained having high melting temperatures, nevertheless introduces the disadvantage of highly disorganizing the crystalline structure of the copolyamide. Thus, for a 6,I/6,T copolyamide of molar composition 70/30, an amorphous copolyamide is obtained having no defined melting point. As the 6,T unit content is increased, crystalline copolyamides are obtained having a high melting point but, however, their crystallinity rapidly falls for 6,T unit contents less than 60% molar. Similarly, 6,I/6,T copolyamides experience a drop of melting point to 215.degree. C., for a 70/30 molar makeup.
It is moreover seen that the glass transition temperature for these copolyamides depends on the melting temperature, thus in order to obtain a glass transition temperature of 140.degree. C. it is necessary to aim for a copolyamide melting points higher than or equal to 345.degree. C.
EP-0 299 689 discloses copolyamides containing the 6,T unit in which the 6,I unit is present in variable proportions as indicated in the table below:
______________________________________ 6,T/6,I molar crystallinity composition % Tf Tg (%) ______________________________________ 65/35 307 119 14 70/30 325 126 25 80/20 345 134 29 ______________________________________
EP-0 299 444 discloses copolyamides in which an aliphatic lactam type unit has undergone copolycondensation with the 6,T unit. However, introducing these aliphatic units decreases the glass transition temperature. In practice, there is a limitation to glass transition temperatures lower than 130.degree. C.; moreover, these aliphatic lactam groups lead to a loss of mechanical properties of the material: of HDT, of flexural modulus, etc. A 6,T/6,I/6,N26 terpolyamide of composition 58/32/10 is exemplified in U.S. Pat. No. 4,246,395. It is however compared to the 6,T/6,I/6,BOB copolyamide of the same composition and T.sub.g and T.sub.M values (135.degree. C./285.degree. C.-133.degree. C./285.degree. C. respectively). However, the 6,N26 unit is considered as a third monomer. Only 6,I/6,T copolyamides (of the copolyamide family including the 6,T unit) are known to have pronounced crystallinity throughout the composition range [see Isomorphous replacement in copolyamide systems: adipic and terephthalic acid, Yu et al., Journal of Polymer Science, Vol. XVII, p. 249-257 (1960)]. However, the latter have glass transition temperatures lower than 120.degree. C.; moreover, the 6,6 unit (originating from condensation of hexamethylene diamine with adipic acid) is well known for its lack of thermal stability, which limits its use. Thus, one frequently finds associated with this 6,6 unit, as a third component, the abovesaid 6,I/6,T or lactam/6,T copolyamides.
This particular feature, attributed to isomorphism of the constituent units of 6,6/6,T copolyamide, is well known to workers in the field and there are many publications covering it; it is attributed to co-crystallinity between the 6,6 and 6,T units. It is also known that this same tendency is found in analogous copolyamides originating from another aliphatic diamine having a longer chain (for example 8,I/8,T) (see: The p-phenylene linkage in linear high polymers: some structure property relationships. Edgar et al., Journal of Polymer Science, Vol. VIII, No. 1, p. 1-22).
Japanese examined patent application (Kokoku) serial No. 5-18877 issued on Mar. 15, 1993 discloses a polyamide, useful as a hot-melt adhesive composition, consisting of aromatic dicarboxylic acid units and C.sub.10 aliphatic alkylenediamine units, said copolymers being however amorphous, which is surprisingly not the case for the copolymers according to the invention.